The present invention relates to the field of electrical testing and measurement, including the use of a sampling oscilloscope and a time domain reflectometer. In particular, this invention relates to an apparatus and method for generating a fast step voltage waveform that is, a step voltage waveform having a low rise time.
A time domain reflectometer ("TDR") is an instrument designed to indicate and to measure reflection characteristics of a transmission system connected to the instrument. A TDR consists of four basic components: (1) a step generator (acting as a signal source), (2) a strobe pulse generator, (3) a sampling device, and (4) a variable delay generator. The TDR produces a step signal and sends it to a system under test. The TDR monitors the step-signals entering the system under test and any reflected transient signals. These signals are displayed on the step-signals on an oscilloscope equipped with a suitable time-base sweep.
As stated above, in a TDR, a step signal is sent to a circuit under test. If there is an impedance mismatch, a signal will be reflected back, and the TDR circuitry will check the reflected signal to determine what type of device is being tested. By measuring a reflection time of the returned signal, defined as the time between a step signal transmission and the receipt of a reflection, one can tell where an impedance mismatch occurs and the extent of the impedance mismatch. This information is derived form a reflection ratio and the reflection time. The reflection ratio is related to the impedance of the device tested and the standardized output impedance of the TDR.
The incident step is generated by a step generator and sent through a transmission line to a connector, and ultimately to the circuit under test. If there is a perfect impedance match there will be no reflection. A display on the TDR will show any reflection of the step signal superimposed on the incident step. If there is a capacitive or inductive mismatch, the reflected signal will have a characteristic shape such as to allow determination of the reactive parameters of the mismatch. A TDR circuit can either display transmission current or voltage at the back termination, the difference being in the displayed polarity of the reflections with respect to the generated step signal.
In a TDR, the lower the risetime of the step generated by the step generator, the more information is sent back from the device being tested since a lower risetime step has more high frequency energy. The lower rise time means that it takes a very short time for the signal to change from its low state to its high state, that is to "step up" to the higher state.
Existing fast and super-fast step generators have employed Josephson junctions and drivers which utilize a single parallel magnetic control line. A fast or super fast step generator can be considered to be a step generator having a rise-time of less than 10 picoseconds. Josephson junction devices utilize the principles of superconductivity, tunneling, and Josephson effects. Superconductivity manifests itself as zero electrical resistance. In order to utilize those principles the devices must be operated at very low temperatures thereby guaranteeing the superconductive properties desired.
A Josephson junction has a current-voltage characteristic similar to that of a tunnel diode. Both the Josephson junction and the tunnel diode have a critical current at which point there is a sudden transition to a gap voltage, also known as the "voltage state." Beyond the gap voltage or voltage state, current rises with increasing voltage. Both the Josephson junction and the tunnel diode current-voltage characteristics display hysteresis for the forward and return paths in a plot of those characteristics.
In the Josephson junction, below the critical current the junction performs as a zero resistance line. At the critical current the junction becomes resistive and current flowing through the junction produces a gap voltage across the junction.
In a tunnel diode there is an operation region according to its I/V (current/voltage) characteristic curve where there is instability, known as a negative resistance region. At the critical current the voltage across the diode increases by an amount denoted the gap voltages. Above this critical current the diode operates in a resistive fashion.
For a niobium Josephson Junction, the voltage state (i.e., the gap voltage) is typically 2.8 mV, however a 2.8 mV step is too small for a practical step in a TDR. In order to increase the size of the step (the amplitude of the step) a plurality of Josephson junctions or tunnel diodes can be connected in series. Problems may be encountered in connecting Josephson junctions or tunnel diodes in series. There has been no way to assure that all of the Josephson junctions or tunnel diodes will have exactly the same critical current ("I.sub.c "). That is, there has been no way to assure that all of the Josephson junctions or tunnel diodes in a given series connection will switch at the same time. If the Josephson. junctions or tunnel diodes do not switch at the same time, a signal produced by the series connection will not be a perfect step. Instead, the signal will be a collection of small steps which will produce an imperfect step.
An example of an existing circuit for producing a step is illustrated in "A Superconducting Josephson Junction Time Domain Reflectometer with Room Temperature Access", S. R. Whiteley, GKG Hohenwarter, and S.M. Faris IEEE Trans. Mag. March 1987 (Proc. Applied Superconducitivity Conf. Baltimore 1986). The circuit includes a drive circuit structure and a step generator that includes a series of Josephson junctions. The drive circuit provides a quick rising current that is transferred to the step generator which is coupled to the drive circuit by a resistance and an inductance.
There are a number of problems with this circuit. First, it requires very tight matching of critical currents or poor switching will result. Second, it requires complex drive circuitry. Both of these factors reduce yield. The probabilistic distribution of critical currents limits expandability and reduces operation margins. Third, the circuit requires insulated magnetic control lines, thus additional insulator and superconductor layers are necessary in processing.
In addition, the circuitry does not have good expansibility. More expansibility, that is the ability to add junctions to the circuitry would result in more amplitude in the output. In other steps generators the circuitry will possibly encounter more difficulties if more junctions are added to that circuitry.
In short, the existing step generator circuitry is sensitive to the imbalance of the driver structure, which results in poor switching, narrow operating margins, and variations of the step wave form obtained.